The present invention relates generally to television receiver sound systems and particularly concerns an improved technique for reducing objectional "buzz" in the reproduced sound signal of an intercarrier sound television receiver.
Intercarrier sound systems are currently used almost exclusively in television receivers. The intercarrier technique for reproducing a sound signal in a television receiver is based on the fact that the frequency modulated sound carrier and the amplitude modulated picture carrier of a broadcast television signal are always spaced from each other by a fixed amount, such as 4.5 MHz in the United States. This characteristic of the transmitted sound and picture carriers allows the two signals to be heterodyned in a television receiver, normally in the video detector stage following IF filtering and amplification, to produce a fixed 4.5 MHz intercarrier sound signal which is subsequently processed in the receiver's intercarrier and audio circuits.
Although intercarrier sound television systems of the foregoing type provide a number of advantages, they are also subject to a number of undesirable phenomena. One such undesirable phenomenon is known as incidental carrier phase modulation (ICPM) which may result in the reproduced sound signal being characterized by an objectionable "buzz". ICPM distortion can result from a number of sources all of which have the affect of imparting undesired video frequency phase modulation onto the picture carrier, which phase modulation is subsequently transferred to the 4.5 MHz intercarrier sound signal in the receiver. The phase modulation so transferred to the 4.5 Mhz intercarrier sound signal results in "buzz" distortion of the reproduced sound signal for phase modulation frequencies that cover a range less than twice the range of the composite baseband audio signal. For example, level dependent phase modulation of the picture carrier by the video signal frequently occurs in broadcast transmitters although this problem can be somewhat controlled by providing suitable correction circuits at the transmitter. The tuning stage of most television receivers may also introduce some ICPM primarily due to the operation of the automatic frequency control circuits and due to reverse mixer feedthrough. Fortunately, the ICPM produced by the tuner is imparted equally to both the picture and sound carriers and therefore normally cancels when the two carriers are heterodyned.
Another source of ICPM results from the so-called Nyquist slope characterizing the IF response of a television receiver about the picture carrier. Television signals are transmitted using a technique referred to as vestigial-side-band transmission wherein the lower picture frequencies (up to about 0.75 MHz) are transmitted as a double-side-band signal and the higher picture frequencies (between about 1.25 MHz and 5.0 MHz) above the picture carrier are transmitted as a single-side-band signal. To compensate for the linear amplitude distortion that would otherwise be produced by the vestigial-side-band transmission of the picture signal, the IF response of the receiver is tailored to reduce the received double-side-band component by an average of fifty percent relative to the received single-side-band component. This is done by imparting maximum IF response to the single-side-band frequencies and by imparting approximately fifty percent of maximum IF response to the IF picture carrier while substantially linearly increasing the IF response therefrom to maximum for one side-band of the double-side-band component and substantially linearly reducing the IF response therefrom to zero for the other side-band. While this IF response characteristic effectively compensates for linear amplitude distortion resulting from the vestigial-side-band nature of the television signal transmission, it also promotes video frequency ICPM of the picture carrier by effecting different gain characteristics for the two side-bands of the picture carrier double-side-band component. As previously explained, this ICPM of the picture carrier is subsequently transferred to the 4.5 MHz intercarrier sound signal and can result in an objectionable "buzz" in the reproduced audio signal. This audio "buzz", while even noticeable in monaural television sound reproduction systems, is particularly objectionable in the case of stereo sound systems which are designed to strongly detect the increased frequency components of the composite stereo baseband signal.
It is therefore a basic object of the present invention to provide an improved intercarrier sound television receiver.
It is a more specific object of the invention to provide a facility in an intercarrier sound television receiver for reducing the effect of ICPM introduced onto the IF picture carrier by the Nyquist slope of the IF response of the receiver.
It is a further object of the invention to provide a SAW IF filter for use in an intercarrier sound television receiver for providing an improved IF response characteristic adapted for eliminating Nyquist slope induced "buzz" from the audio reproduced by the receiver.